Fix: Allow resolving house parent scope (town) of unbuilt houses.

[PeterN]

Motivation / Problem

If a town house uses parent scope then trying to display it in the House Picker will crash, as TownScopeResolver does not handle a non-existent town.

Description

In TownScopeResolver, return unavailable variable result if town is not set, instead of nullptr dereference.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker